The invention relates to a device for the magnetic recording and/or playback of signals on a flexible recording medium by means of a magnetic transducing head, and more particularly to such a device having transport and guide means for bringing successive parts of the flexible recording medium in physical contact with a surface of the head, the surface being curved and comprising a non-magnetic transducing gap.
The invention further relates to a magnetic transducing head for the recording or playback of signals on a flexible recording medium which is passed over a curved contact face of the transducing head, the transducing head having a non magnetic transducing gap which intersects the direction of movement of the recording medium.
For a successful recording and playback of signals on flexible record carriers particularly at high densities, it is important in order to achieve a minimal signal loss that, in the area of the transducing gap, the spacing between the transducing head and the recording medium is minimum and preferably is zero. The area where the last-mentioned requirement is satisfied, (the so-called contact face) is here defined as the area where the distance between head and record carrier is smaller than approximately 3.times. the combined roughness of their surfaces (the transducing head then is of the "in-contact" type). At larger values, there is assumed to be an air film between the surfaces (the transducing head then is of the "fluid foil bearing" type).
In devices for recording and playback signals operating with flexible recording media, the interaction of a large number of parameters has a considerable effect on the predictability and possibility of maintaining such a minimum distance between head and recording medium. The parameters which influence this distance comprise inter alia the contour of the surface of the head facing the recording medium, the relative velocity between transducing head and recording medium, the tape tension, and the medium parameters such as the thickness, the modulus of elasticity and the transverse contraction (Poisson's ratio), which together determine the bending stiffness.
Magnetic heads are normally provided with circular profiles having a constant radius. The thickness h.sub.o of the air film between the head surface and the recording medium may be determined according to the theory of A. Eshel and H. G. Elrod jr. (see: "The Theory of the Infinitely Wide, Perfectly Flexible Foil, Bearing, Journal of Basic Engineering, Transactions A.S.M.E., series D, Vol. 87, 1965, pp 831-836) as follows: ##EQU1## where: .eta.=viscosity of the medium in which the recording medium is moving (usually air);
U=relative velocity of recording medium relative to the head; PA1 T=tape tension (=pulling force/tape width) PA1 R=radius of the head profile; PA1 K.sub.H =model constant dependent on the angle at which the tape is wrapped about the head, width and bending rigidity of the recording carrier.
One possibility of minimizing h.sub.o for a given recording and playback device and using a given magnetic tape as a recording medium (in which the viscosity .eta. of the medium in which the recording medium is moving, its velocity U, the tape tension T and the constant K.sub.H are fixed) is to make the radius of curvature R small. However, this has disadvantages.
The local contact pressure p for circular cylindrical profiles may be written as: EQU p=T/R
where T=tape tension; R=radius of the profile.
This means that as the radius of the profile decreases, the local contact pressure p becomes larger and this means that the detrition rate (.delta.h)/(.delta.l), which may be written as: ##EQU2## where h=local depth of detrition, l=passed tape length, K and .alpha. are model constants,
increases with a reduction of R.
The gap depth is one of the most important parameters which vary during the detrition process. For a maximum transducing function, a small gap depth is desired. The larger the detrition rate, the sooner the depth of detrition will exceed the gap depth whereby the head becomes useless.
Although the maximum duration of use of the head can be extended by designing the head with a larger gap depth, this will be at the expense of the quality.
Thus, it is impossible with one degree of freedom (the radius R) to minimize both the thickness of the air film and the contact pressure independently of each other.